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Severe root-knot nematode soil infestations can produce abundant root galling and significantly reduce the uptake of nutrients as well as yield.

Over the past several years the number of fields observed to contain root-knot nematodes has increased across Mississippi. The best way to determine the presence of a nematode infestation (whether reniform or root-knot or soybean cyst nematode) is to soil sample. Soil samples can then be submitted to the diagnostic laboratory for analysis to determine 1) the specific nematode species present and 2) the number of nematodes present in the soil sample submitted. Not only is soil sampling important from a nutrition standpoint, but determining the specific nematode present in a field can help determine the variety or rotational crop that best suits the field situation. Presently, commercially available varieties do not contain resistance to the reniform nematode. However, some tolerance within soybean varieties to the root-knot nematode does exist.

In 2015, a version of the MSU soybean OVT was planted in a field with a known infestation of the root-knot nematode (RKN) in New Albany, MS. A total of 133 entries were planted consisting of the RoundUp Ready entries, minus the conventional (MG IV and V) and LibertyLink (MG IV and V) varieties. The field was located in northeastern Union County and had a history of root-knot nematode. Prior to the 2015 season soybean plants from this particular field had been observed to contain a substantial amount of root galling as a result of the RKN.

Immediately prior to harvest, roots were removed from rows 1 and 4 with a shovel and rated on a scale of 0-5, whereby 0=no root galling and 5=severe root galling of the taproot as well as lateral roots. Yield was harvested from the middle (rows 2 and 3) of each plot to assess the impact of RKN on overall yield.

Among the MG IV early varieties, the lowest yielding variety was Mycogen 5N433R2 with a yield of 32.1 bu/A. Morever, this particular variety had a relatively low root galling rating, on average a 1.2 on a scale of 0-5 indicating low sensitivity to RKN populations. The greatest yielding variety in the MG IV early varieties was Dyna-Gro 31RY45, with a yield of 61 bu/A. The Dyna-Gro variety had a low root galling rating, a 1.0, suggesting the variety has good resistance to the root-knot nematode. The most tolerant variety based on the lowest amount of root galling was Credenz CZ 4181 with a rating of 0.3 suggesting strong resistance; however, the yield was lower than the variety that produced the greatest yield, but was greater than the average of all the MG IV early varieties evaluated. In addition, yield significantly differed between among with a range of 28.9 bu/A between the 30 entries. Root galling also significantly differed among varieties with a low of 0.3 and a high of 4.0. No correlation between yield and rot galling was observed for this particular set of entries Figure 1 (2015 scatter plots).

Among the MG IV late varieties, the lowest yielding variety was Delta Grow DG 4765 with a yield of 33.7 bu/A. In addition, DG 4765 had one of the greatest root galling ratings (3.2) compared to the other entries suggesting a high level of susceptibility to the root-knot nematode. Even though this particular variety did not have the greatest root galling, the root galling associated with this particular Progeny variety suggests it is extremely susceptible to the RKN. The greatest yielding variety in this particular group was Go Soy 4914GTS with a yield of 63.7 bu/A. In addition, with the greatest yield, 4914 GTS also had the lowest root galling rating, 0.3, suggesting the variety is resistant to the root-knot nematode. The root galling encountered with some of the varieties suggests that some varieties, while good yielders in the presence of RKN, can see sustain a high level of root galling. Yield was significantly different among varieties with a range of 30 bu/A between the 51 entries. Root galling also significantly differed among varieties with a low of 0.3 and a high of 3.9. No correlation between yield and rot galling was observed for this particular set of entries Figure 1 (2015 scatter plots).

Among the MG V early varieties, the lowest yielding variety was Asgrow AG 5233 with a yield of 34.4 bu/A. However, the variety had a root galling rating of 1.9, which was well below some of the varieties with greater root galling observed. The greatest yielding variety was Asgrow AG 5535 with a yield of 62.2 bu/A. However, even though this particular variety produced the greatest yield the root galling that occurred as a result of RKN was still greater than 4, suggesting that the variety can out yield root damage as a result of RKN. But, it should be noted that root galling was only observed at one time during the season. Yield significantly differed among varieties with a range of 42.4 bu/A between the 44 entries. In addition, root galling between varieties significantly differed with a range of 1 to 3.7. No correlation between yield and rot galling was observed for this particular set of entries Figure 1 (2015 scatter plots).

Among the MG V late entries, the lowest yielding variety was NK Brand S59-V9 with a yield of 42.8 bu/A. The greatest yielding variety was NK Brand S58-Z4 with 86.3 bu/A. Both varieties had substantial galling, with the greatest yielding variety having the greatest observed root galling. Yields significantly different among varieties with a range of 43.5 bu/A between the eight entries. In addition, root galling was significantly different among varieties with a low of 0.3 and a high of 2. A statistical correlation between yield and root galling existed for the MG V late varieties as indicated in Figure 1 (2015 scatter plots).

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